Self-interstitial Clusters Research Articles

Oxygen trapping in defect clusters in Fe and FeCr alloy by ion channeling and ab-initio study

Ion channeling experiment and ab-initio density functional theory (DFT) calculations are used to study O interactions with interstitial and vacancy clusters in Fe and FeCr alloy. Chromium and O18 ions are implanted at the same depth in a Fe crystal and annealed at 400 °C. Lattice location of O18 is found to be tetrahedral interstitial site by nuclear reaction analysis (NRA)/channeling and Cr is found at substitutional site with small displacement by particle induced X-ray emission (PIXE)/channeling experiment. DFT calculations predict similar sites of O18 and Cr for trapping in self-interstitial clusters. On the other hand, O18 is implanted in a Fe15at%Cr alloy and annealed at 400 °C. The O18 is found to be displaced 0.6 Å along <100> from the octahedral interstitial site in FeCr alloy. DFT calculations predict similar lattice location of O for trapping in vacancy clusters. The Cr atoms show strong influence on the O trapping at defects in FeCr alloy.

Quantum de-trapping and transport of heavy defects in tungsten.

The diffusion of defects in crystalline materials1 controls macroscopic behaviour of a wide range of processes, including alloying, precipitation, phase transformation and creep2. In real materials, intrinsic defects are unavoidably bound to static trapping centres such as impurity atoms, meaning that their diffusion is dominated by de-trapping processes. It is generally believed that de-trapping occurs only by thermal activation. Here, we report the direct observation of the quantum de-trapping of defects below around one-third of the Debye temperature. We successfully monitored the de-trapping and migration of self-interstitial atom clusters, strongly trapped by impurity atoms in tungsten, by triggering de-trapping out of equilibrium at cryogenic temperatures, using high-energy electron irradiation and in situ transmission electron microscopy. The quantum-assisted de-trapping leads to low-temperature diffusion rates orders of magnitude higher than a naive classical estimate suggests. Our analysis shows that this phenomenon is generic to any crystalline material.

Comparative study of radiation defects in ion irradiated bulk and thin-foil tungsten

In this study, we employ transmission electron microscope (TEM) to analyze radiation defects in helium (He) and krypton (Kr) ions implanted bulk and thin-foil tungsten. For bulk tungsten, subgrains are formed near the surface region under both He+ and Kr+ irradiation. Dislocation loops are observed beyond ion implanted range. These observations are related to self-interstitial atoms (SIAs) diffusion and clustering. Ordered bubbles are formed in He+ implantation, while no cavities are detected in Kr+ irradiation. In thin-foil tungsten, line up of dislocation loops is found mainly aligns along {101} and {112} slip planes. Both 1/2<111> and 〈100〉 dislocation loops are identified. Compared to He+ irradiation, more 〈100〉 loops are detected in Kr+ irradiation due to higher energy collision cascade. Nanocavities are detected in irradiated thin-foil tungsten besides the formation of high density of interstitial loops. The number density of dislocation loops and the volume fractions of cavities are higher in He+ irradiation than in Kr+ irradiation. The differences in nature of radiation defects is attributed to the higher recombination rate of vacancies and interstitials in bulk sample, the significant surface sink effect in thin-foil irradiation and the chemical and physical effect of implanted ions.

Machine-learning interatomic potential for radiation damage and defects in tungsten

We introduce a machine-learning interatomic potential for tungsten using the Gaussian Approximation Potential framework. We specifically focus on properties relevant for simulations of radiation-induced collision cascades and the damage they produce, including a realistic repulsive potential for the short-range many-body cascade dynamics and a good description of the liquid phase. Furthermore, the potential accurately reproduces surface properties and the energetics of vacancy and self-interstitial clusters, which have been long-standing deficiencies of existing potentials. The potential enables molecular dynamics simulations of radiation damage in tungsten with unprecedented accuracy.

Effects of one-dimensional migration of self-interstitial atom clusters on the decreasing behaviour of their number density in electron-irradiated α-iron

ABSTRACTAn in- situ observation using high-voltage electron microscopy (HVEM) showed that the number density of stationary self-interstitial atom (SIA) clusters in electron-irradiated α-iron at room temperature seems to reach its peak value at an early stage of irradiation and continuously decreases at the prolonged irradiation. A thin foil specimen is used in the in situ HVEM experiments; hence, the SIA clusters can annihilate because of their escape to the specimen surfaces and their mutual coalescence through one-dimensional (1D) migration. In this study, we derive analytical models associated with the experimentally revealed 1D migration mechanisms to examine the decreasing behaviour of the cluster number density: (1) trapping of one-dimensionally migrating SIA clusters by impurity atoms within the specimen surfaces and (2) detrapping of stationary SIA clusters from a bounded impurity atom caused by impact with incident electrons. By introducing a simple cluster evolution model that accounts for only the trapping and detrapping processes, the model calculation indicates that the detrapping of the stationary SIA clusters causes the surface annihilation of the liberated SIA clusters, leading to the decrease in their number density. The decreasing behaviour is in closer accordance with the experimental data when setting the impurity concentration in the same order as the estimation from the previous in situ HVEM experiment. This result suggests that the trapping and detrapping of the SIA clusters are the possible underlying processes for the decreasing behaviour.

Evaluation of Cr Concentration Effect on Displacement Cascades in Fe–Cr Alloys with Piecewise Potential

A piecewise potential was constructed with combination of the concentration-dependent EAM (CD-EAM) potential and Ziegler–Biersack–Littmark (ZBL) potential, and the displacement cascades in random Fe–Cr binary alloys with a wide range of Cr concentration from 0 to 20% were simulated to evaluate the effect of Cr concentration on defect formation and evolution during the cascades. The Cr concentration has little or no effect on the fraction of vacancy clusters, the number of surviving Frenkel pairs, or the self-interstitial and vacancy cluster sizes and a slight effect on the peak time and peak defect number, but a considerable effect on the Cr enrichment in the peak and surviving defects in the cascaded Fe–Cr alloys. With the increase in Cr concentration, the Cr enrichment in the peak and surviving defects exhibits a downtrend. The higher enrichment degree in the surviving defects than in the peak defects shows more difficult recombination of Cr SIAs with vacancies.

Infrared spectroscopy studies of localized vibrations in neutron irradiated silicon

We investigate neutron irradiation-induced defects in p-type Czochralski silicon (Cz–Si) subjected initially to heat treatments under high hydrostatic pressure (HTHP), by means of infrared spectroscopy (IR). A pair of bands at 592 and 883 cm−1 arises in the spectra immediately after irradiation and disappears upon isochronal annealing just below 350 °C in as-grown Si, although they disappear at a smaller temperature ~ 280 °C in the HTHP treated Si. Another pair of bands at 535 and 556 cm−1 arises in the spectra at ~ 320 °C and disappears at ~ 430 °C in as-grown Si, although they show a shift in their thermal stability of ~ 50 °C towards lower temperatures in HTHP Si. The activation energies characterizing their annihilation were found smaller in the HTHP Si, for each one of the four bands correspondingly. It is argued that the applied hydrostatic pressure affects the annealing behavior of the bands promoting their annihilation. From the LVM frequency values, the temperature range they appear and their annealing behavior we tentatively correlate them with structures involving self-interstitial clusters, presumably perturbed by an impurity atom. Four other bands at 562, 642, 654 and 678 cm−1 show similar thermal stability arising in the spectra in the course of the isochronal annealing at ~ 250 °C and disappearing at ~ 400 °C, both in as-grown and in HTHP Si. However, the changes exhibited in the values of the activation energies of the bands between the HTHP and the as-grown Si, suggest that may not all of them have exactly the same origin, at least the 678 cm−1 band. The origin of the above family of bands is discussed in regards with previous works reported in the literature. Connection with complexes comprising boron atoms and self interstitials, in short (Bn–SiIm), was considered.

The interactions between rhenium and interstitial-type defects in bulk tungsten: A combined study by molecular dynamics and molecular statics simulations

Tungsten (W) and W-based alloys are the leading candidates for plasma-facing materials (PFMs) in future fusion reactors. However, the high energy neutrons generated in fusion reactions not only result in cascade damages but also cause W transmutation. Both the irradiation defects and transmutation products, mainly rhenium (Re), have serious effects on the service behaviors of W PFMs. In this work, we have systematically investigated the interaction between Re and the self-interstitial atoms, self-interstitial clusters and 1/2<111> interstitial dislocation loops in bulk W using molecular dynamics and statics simulations. It is found that there is a strong attractive interaction between an interstitial W atom and a substitutional Re atom, forming a Re–W dumbbell that migrates 3-dimentionally due to the low migration and rotation energies. The small SIA clusters strongly bind with both the substitutional Re atoms and an interstitial Re atom (Re–W mixed dumbbell), thus decreasing the mobility of these clusters. The strong attractive interaction between a Re atom and a 1/2<111> interstitial dislocation loop occurs when the Re atom is located at the core of the loop, and also, their interaction distance along <111> direction is large. The mobility of the 1/2<111> interstitial dislocation loop decreases progressively with increasing Re concentration.

Collision cascades overlapping with self-interstitial defect clusters in Fe and W

Overlap of collision cascades with previously formed defect clusters become increasingly likely at radiation doses typical for materials in nuclear reactors. Using molecular dynamics, we systematically investigate the effects of different pre-existing self-interstitial clusters on the damage produced by an overlapping cascade in bcc iron and tungsten. We find that the number of new Frenkel pairs created in direct overlap with an interstitial cluster is reduced to essentially zero, when the size of the defect cluster is comparable to that of the disordered cascade volume. We develop an analytical model for this reduced defect production as a function of the spatial overlap between a cascade and a defect cluster of a given size. Furthermore, we discuss cascade-induced changes in the morphology of self-interstitial clusters, including transformations between and dislocation loops in iron and tungsten, and between C15 clusters and dislocation loops in iron. Our results provide crucial new cascade-overlap effects to be taken into account in multi-scale modelling of radiation damage in bcc metals.

Deformation-free nanotwin formation in zirconium and titanium

An irradiation-induced nanotwinning mechanism in zirconium and titanium is proposed, based on molecular dynamics simulations. Atomistic configurations before and after nanotwinning are characterized in Zr, Mg and Ti, using four different semi-empirical potentials. An elliptical self-interstitial cluster is identified as the precursor to nanotwinning. These elliptical self-interstitial clusters spontaneously formed and transformed into nanotwins in two cases—Zr and Ti—but not in Mg. The nanotwins are crystallographically equivalent to 112¯1112¯6¯ tensile deformation twins, which are experimentally observed in Zr and Ti. We noted that nanotwins appeared in the hcp materials with low 101¯0 prismatic plane stacking fault energy (SFE), relative to their basal extrinsic SFE.

Energetics, kinetics and dynamics of self-interstitial clusters in bcc tungsten

ABSTRACTThe configuration, slipping and rotation of self-interstitial atoms cluster along <111> crystal orientation with different sizes in a tungsten are investigated systematically with molecular dynamics. It is found that (I) the SIA clusters with high symmetry are always favoured; (II) the SIA clusters can undergo one-dimensional fast migration along <111> direction, and their migration barriers are no more than 0.07 eV, which is expected due to the strong interaction in the SIA clusters; (III) the rotation energy barriers of the SIA clusters are rather high and they are basically positively correlated with the size of the cluster. For example, the reorientation barrier is 0.66 eV for 1 SIA, 1.2–1.8 eV for SIAn (2 ≤ n ≤ 5) clusters and over 2.7 eV for SIAn (6 ≤ n ≤ 7) clusters. Compared with slipping of SIA clusters, is an infrequent event, especially for larger SIAs cluster, the vast majority SIAs cluster would have already recombination with vacancies or annihilates at surface and grain boundary through slipping before rotation, which explained that there are very low density of SIAs cluster found in the experiment.

Cluster dynamics simulation of the flux effect for neutron irradiated pure iron

ABSTRACTThe effect of flux in a wide range from 1.39×10−10 to 1.39 ×10−3 dpa/s has been studied by the cluster dynamics (CD) simulation for pure iron neutron irradiated at a temperature of 300°C for the two fluences. The critical flux values which mark the regimes of flux dependence of average size, number density and irradiation induced hardening for vacancy clusters (VC) and self-interstitial atoms clusters (SIAC) are found. It was found that the effect of neutron irradiation on average size of SIAC was similar for low and high fluxes in the case of the same fluence.

Radiation Defects in Aluminum. Simulation of Primary Damage in Surface Collision Cascades

Molecular dynamics method has been applied to simulate collision cascades initiated by primary knock-on atoms (PKAs) with energy EPKA = 5, 10, 15, and 20 keV at temperatures T = 100, 300, and 600 K on the aluminum surface. A series of 48 cascades has been simulated for each pair of parameters (EPKA, T), providing a representative statistical sampling. The number of Frenkel pairs NFP, the fraction of vacancies evac and self-interstitial atoms (SIA) eSIA in clusters of point defects, the average size of vacancy 〈Nvac〉 and self-interstitial 〈NSIA〉 clusters, the average number of vacancy 〈Yvac〉 and self-interstitial 〈YSIA〉 cluster per cascade yield, and the average time τc of cascade relaxation as a function (ЕPKA, T) have been found. The level of primary damage, 〈evac〉, 〈eSIA〉, 〈Nvac〉, 〈NSIA〉, 〈Yvac〉 and 〈YSIA〉 have been found to be higher in surface cascades than those in displacements cascades in the bulk of material under the same simulation conditions. The morphology of surface cascades and the spatial separation of self-interstitial atoms and vacancies has been studied. Icosahedral self-interstitial clusters have been identified in displacement cascades in aluminum for the first time.

Diffusion kinetic of hydrogen in CH3O-molecular-ion-implanted silicon wafer for CMOS image sensors

The association and dissociation behavior of hydrogen in the implanted region of a CH3O cluster were investigated for high-performance CMOS image sensors. Two hydrogen peaks were observed in the CH3O-implanted region after epitaxial growth and heat treatment. The difference in the depths of the two peaks accords with the difference in the depth of carbon-cluster-related and new extended stacking fault defects. Thus, we calculated the activation energies of the association and dissociation of hydrogen, assuming a dissociation reaction for the first peak, formed at a small depth from the surface of the silicon substrate, and a simple reversible reaction for the second peak, formed at a large depth from the surface of the silicon substrate. The dissociation activation energy corresponding to the first peak was 0.75 ± 0.03 eV. This activation energy indicates that the hydrogen associated with the first peak forms a binding state with a carbon and silicon self-interstitial cluster (C/I cluster). On the other hand, the binding energy corresponding to the second peak was calculated to be 0.78 eV, which was close to the C–H2 binding energy. Consequently, the CH3O-implanted region forms a binding state with hydrogen and a C/I cluster or a C–H2 binding state.

A tungsten-rhenium interatomic potential for point defect studies

A tungsten-rhenium (W-Re) classical interatomic potential is developed within the embedded atom method interaction framework. A force-matching method is employed to fit the potential to ab initio forces, energies, and stresses. Simulated annealing is combined with the conjugate gradient technique to search for an optimum potential from over 1000 initial trial sets. The potential is designed for studying point defects in W-Re systems. It gives good predictions of the formation energies of Re defects in W and the binding energies of W self-interstitial clusters with Re. The potential is further evaluated for describing the formation energy of structures in the σ and χ intermetallic phases. The predicted convex-hulls of formation energy are in excellent agreement with ab initio data. In pure Re, the potential can reproduce the formation energies of vacancies and self-interstitial defects sufficiently accurately and gives the correct ground state self-interstitial configuration. Furthermore, by including liquid structures in the fit, the potential yields a Re melting temperature (3130 K) that is close to the experimental value (3459 K).

Effect of post-implantation annealing on Al–N isoelectronic trap formation in silicon: Al–N pair formation and defect recovery mechanisms

The effect of post-implantation annealing (PIA) on Al–N isoelectronic trap (IET) formation in silicon has been experimentally investigated to discuss the Al–N IET formation and implantation-induced defect recovery mechanisms. We performed a photoluminescence study, which indicated that self-interstitial clusters and accompanying vacancies are generated in the ion implantation process. It is supposed that Al and N atoms move to the vacancy sites and form stable Al–N pairs in the PIA process. Furthermore, the PIA process recovers self-interstitial clusters while transforming their atomic configuration. The critical temperature for the formation/dissociation of Al–N pairs was found to be 450 °C, with which we describe the process integration for devices utilizing Al–N IET technology.

W and X Photoluminescence Centers in Crystalline Si: Chasing Candidates at Atomic Level Through Multiscale Simulations

Several atomistic techniques have been combined to identify the structure of defects responsible for X and W photoluminescence lines in crystalline Si. We used kinetic Monte Carlo simulations to reproduce irradiation and annealing conditions used in photoluminescence experiments. We found that W and X radiative centers are related to small Si self-interstitial clusters but coexist with larger Si self-interstitials clusters that can act as nonradiative centers. We used molecular dynamics simulations to explore the many different configurations of small Si self-interstitial clusters, and selected those having symmetry compatible with W and X photoluminescence centers. Using ab initio simulations, we calculated their formation energy, donor levels, and energy of local vibrational modes. On the basis of photoluminescence experiments and our multiscale theoretical calculations, we discuss the possible atomic configurations responsible for W and X photoluminescence centers in Si. Our simulations also reveal that the intensity of photoluminescence lines is the result of competition between radiative centers and nonradiative competitors, which can explain the experimental quenching of W and X lines even in the presence of the photoluminescence centers.

Kinetic Monte Carlo model for 1-D migration in a field of strong traps: Application to self-interstitial clusters in W-Re alloys

In this work, we develop a kinetic Monte Carlo (KMC) scheme to describe 1-D migration in a field of traps. The model is parameterized solely on static calculations but is capable to predict the evolution of the system at finite temperature. The model is parameterized to describe the migration of self-interstitial clusters in the W-Re system that is of particular interest for fusion applications. The developed KMC model is benchmarked against molecular dynamics simulations and is found suitable to replace the latter at a fraction of the computational cost. We found that the presence of even traces of Re (starting from 0.1%) has a significant impact on the mobility of small 1-D migrating self-interstitial clusters. In addition to the computational scheme, the obtained results elucidate the impact of Re on void swelling and the suppression of void lattice formation under neutron irradiation of tungsten.

Ultrafast Generation of Unconventional {001} Loops in Si.

Ultrafast laser annealing of ion implanted Si has led to thermodynamically unexpected large {001} self-interstitial loops, and the failure of Ostwald ripening models for describing self-interstitial cluster growth. We have carried out molecular dynamics simulations in combination with focused experiments in order to demonstrate that at temperatures close to the melting point, self-interstitial rich Si is driven into dense liquidlike droplets that are highly mobile within the solid crystalline Si matrix. These liquid droplets grow by a coalescence mechanism and eventually transform into {001} loops through a liquid-to-solid phase transition in the nanosecond time scale.

Study of point defects diffusion in nickel using kinetic activation-relaxation technique

Point defects play a central role in materials properties. Yet, details regarding their diffusion and aggregation are still largely lacking beyond the monomer and dimer. Using the kinetic Activation Relaxation Technique (k-ART), a recently proposed off-lattice kinetic Monte Carlo method, the energy landscape, kinetics and diffusion mechanisms of point defect in fcc nickel are characterized, providing an exhaustive picture of the motion of one to five vacancies and self-interstitials in this system. Starting with a comparison of the prediction of four empirical potentials — the embedded atom method (EAM), the original modified embedded atom method (MEAM1NN), the second nearest neighbor modified embedded atom method (MEAM2NN) and the Reactive Force Field (ReaxFF) —, it is shown that while both EAM and ReaxFF capture the right physics, EAM provides the overall best agreement with ab initio and molecular dynamics simulations and available experiments both for vacancies and interstitial defect energetics and kinetics. Extensive k-ART simulations using this potential provide complete details of the energy landscape associated with these defects, demonstrated a complex set of mechanisms available to both vacancies and self-interstitials even in a simple environment such as crystalline Ni. We find, in particular, that the diffusion barriers of both vacancies and interstitials do not change monotonically with the cluster size and that some clusters of vacancies diffuse more easily than single ones. As self-interstitial clusters grow, moreover, we show that the fast diffusion takes place from excited states but ground states can act as pinning centers, contrary to what could be expected.